This invention pertains to reverse osmosis or nanofiltration membranes used for desalination of water or removal of other solutes from liquids. There is an ever-increasing need for membranes which can perform removal of solutes at lower operating pressures, thus reducing energy requirements. The goal of development efforts is to increase the water permeability of such membranes while limiting the amount of salt or other solute passage. U.S. Pat. Nos. 4,765,897 and 4,812,270 describe the use of strong mineral acids followed by treatment of rejection-enhancing agents to produce low pressure water softening membranes. However, these membranes are limited to 95 percent or less rejection of magnesium sulfate and even lower rejection of sodium chloride. A further disadvantage of the prior art processes is the requirement for an additional polymeric treatment step to repair the damage to the salt rejecting layer of the membrane caused by the strong mineral acid treatment step.
Desirable therefore are high flux, higher solute rejection membranes operable at very low pressures which can be prepared by an efficient one-step treatment process. Specifically desired in nanofiltration applications is a membrane which has a flux of at least 15 gallons per square foot per day (gfd) and a sodium chloride rejection of at least about 20% when pressurized to 75 pounds per square inch (psi) using 0.05 weight percent sodium chloride. When tested on 0.2 weight percent magnesium sulfate under this pressure rejection should typically be at least about 90%.